The presently disclosed subject matter relates to a measuring apparatus and a measuring method, in particular, a measuring apparatus and a measuring method related to respiration-induced fluctuations in blood pressure.
Pulsus paradoxus (hereinafter, “PP”) is a respiration-induced fluctuation in systolic blood pressure exceeding 10 mmHg. PP is used in determining asthma, chronic obstructive pulmonary disease (COPD), cardiac tamponade, and the like (see, e.g., Bandinelli G, et al., “Pulsus Paradoxus: an inderused tool,” Intern Emerg Med (2007) 2: p. 33-35). In the National Asthma Education and Prevention Program (NAEPP), a guideline for acute asthma, for example, PP exceeding 12 mmHg is defined as a level at which treatment in a hospital is required (see, e.g., James Rayner, et al., “Continuous Noninvasive Measurement of Pulsus Paradoxus Complements Medical Decision Making in Assessment of Acute Asthma Severity,” Chest. 2006 September; 130(3): p. 754-765).
As described above, the detection of PP is considered to be important in determining asthma and the like. Therefore, accurate detection of a measurement value of a respiration-induced blood pressure fluctuation is very important. Hereinafter, some related art methods for measuring respiration-induced blood pressure fluctuations will be described.
A first example is a measuring method using a Korotkoff sound. In this method, when the blood pressure is to be measured by the auscultatory method, the respiration-induced blood pressure fluctuation is calculated by measuring the difference, between the expiratory and inspiratory phases, of the cuff pressure at which a Korotkoff sound begins to be heard. This method, however, has problems in that it requires a skill of a doctor making a diagnosis and that accuracy is low (see, e.g., James Rayner, et al. and also Gregory D. Jay, et al., “Analysis of Physician Ability in the Measurement of Pulsus Paradoxus by Sphygmomanometry,” Chest. 2000; 118(2): p. 348-352).
James Rayner, et al. also discloses a method for measuring a respiration-induced blood pressure fluctuation by using a non-invasive continuous blood pressure monitor. In this method, a tonometry method and a volume compensation method that are practically used in a continuous blood pressure monitor are used. However, the tonometry method and the volume compensation method require an apparatus having a complicated sensor structure and a complex pressure control, and such an apparatus is very expensive. Therefore, the tonometry method and the volume compensation method are not being widely used.
Dale W. Steele, et al. discloses a method for measuring a respiration-induced fluctuation by using the peripheral volume pulse wave acquired by a pulse oximeter (Dale W. Steele, et al., “Continuous Noninvasive Determination of Pulsus Paradoxus: A Pilot Study,” ACADEMIC EMERGENCY MEDICINE, October 1995, VOL. 2/No. 10, p. 894-900). In this method, however, a ratio of the respiration-induced blood pressure fluctuation to the pulse wave amplitude is obtained, but a pressure equivalent measurement value is not obtained. Because the respiration-induced blood pressure fluctuation is not calculated as a pressure equivalent measurement value, this method is difficult to be used in determining diagnostic strategies by doctors. Moreover, the respiration-induced fluctuation calculated by this method has less correlation with the respiration-induced fluctuation calculated by an invasive method, as compared with the respiration-induced fluctuation calculated by a commercially available continuous blood pressure monitor using the volume compensation method.
JP2002-524177A discloses an apparatus for calculating a respiration-induced fluctuation of the volume pulse wave as a pressure equivalent measurement value. In this apparatus, pulse-wave waveforms in which the measurement values of PP are given are predefined (see, e.g., paragraph 0039 of JP2002-524177A). Then, the apparatus compares the predefined pulse-wave waveforms with a pulse-wave waveform measured from the subject, thereby calculating a measurement value of PP.
That is, the apparatus of JP2002-524177A is configured such that pulse-wave waveforms correlated with the respective measurement values of PP are predefined and then stored. However, it is difficult to predefine the pulse-wave waveforms and their associated measurement values of PP. In other words, a prior calibration is required, but it is difficult to perform such a calibration.